Partial combustion or gasification of coal involves reacting the coal at elevated temperatures and possibly elevated pressures with a limited volume of oxygen, the reaction preferably being carried out in the presence of additional agents such as steam, carbon dioxide, or various other materials. Gasification of coal produces a gas, known as synthesis gas, that contains mostly carbon monoxide and hydrogen. Also produced are varying quantities of other gases, such as carbon dioxide and methane, and various liquid and solid materials.
Many coals contain, in addition to carbon and hydrogen, varying quantities of inorganic incombustible material. This material ultimately becomes a particulate by-product of the process of oxidation, and, depending on characteristics such as density and size of the particular particle, and the reactor configuration and conditions, may undergo a rough separation in the reactor into particles respectively called "flyash" (lighter) and "slag" (denser). In a vertically disposed gasification zone or reactor, the flyash particles are removed overhead, while the denser materials collect as a molten slag, often including separated iron, in the hearth of the reactor. From the hearth of the reactor the slag is discharged downward through an outlet or orifice, referred to as a slag tap, into a water bath.
This slag, because it is derived from a "reducing" atmosphere, tends to be different in composition and properties from slag normally associated with combustion boilers where a fully oxidizing atmosphere is utilized. For example, the slag from processes for partial combustion of coal may contain elemental iron and sulfides, components not normally associated with boiler slag.
The slag may have undesirable properties insofar as its ultimate disposal is concerned. In particular, it may contain undesirable species such as arsenic, selenium, and the aforementioned sulfides, and, if the slag is to be utilized, or disposed of as land fill, it must be in a form which does not release such materials readily to the environment. The sulfides present are particularly troublesome since, upon contact with strongly acidic material, hydrogen sulfide will be evolved. Accordingly, the slag removed from the bottom of the water bath in partial combustion processes will normally undergo further processing before use or disposal. The invention addresses particularly the problem of the evolution of H.sub.2 S from such slag.